Support for "floppy" structure in stainless steel

Imagine a part which looks like, oh, maybe a model boat propeller.



And imagine it is laying flat on the table, as in the picture.

It has a center hub, and blades which are cantilevered out in the radial direction. The blades are thick near the root, but taper to a thin edge. But also, when the prop sits on the table, the blades are unsupported.

If such a part is made in stainless steel, I fear that the blades would break or sag in the sintering and infusion process. So perhaps I can add small support features which will be removed later.

My question: What does the part sit on during the sintering process? A flat table? Or some sort of grid or screen?

If it is a flat table, my supports can be pins- like skinny table legs.
If it is a screen, then the supports need big feet.

Another question: How does the infusion process work? Is the part sintered at high temperature first? And afterward, the bronze is introduced in some way? (sorry if this is described somewhere else, I couldn't find it!)

Thanks,
Pat M

During the sintering process, the growing metal part sits on a bed of powdered stainless steel; one can think of the part as floating in a pool of powder whose surface is being selectively sintered.

During the infiltration process, the part is surrounded in a bath of small pellets of support material whose composition has not been revealed, to my knowledge.

I have noticed significant sag in my metal prints relative to SLS nylon prints (WSF). However, this sag is a fraction of a millimeter at most, nothing like one would expect if parts were unsupported.

Takeaway message: tolerances are worse in metal than in WSF (and of Shapeways parts, reportedly Grey Robust has the best dimensional stability).

OK, after the first post, I found the video at https://www.shapeways.com/materials/stainless_steel .

I can see how the support medium is poured in place around the part... but I can't see what the bottom of the container looks like... maybe flat? But I am still thinking that a removable support structure on the part would help guarantee minimal sag...

Hi Guys
The "support material" is a flavor of granular alumina. The containers used during sintering are flat bottomed. A bed of the alumina is first laid down and the green parts are nestled into the alumina. To these parts, small sticks of printed media are now attached. These sticks are called "stilts" but think of them more like the gates attached to a part that would be produced by traditional casting methods. The stilts are the conduit (like a wick) for the bronze infiltrants. These stilts are cut away after sintering. With me so far? O.K. now a calculated amount of bronze material is piled onto one end of the stilt. More alumina is now added until the parts are covered. This prepared container of parts now goes into the vacuum furnace for sintering. Because heat is applied in a vacuum the bronze is able to withstand the high temperatures required for the stainless particles to sinter. As the stainless particles are bonding to each other the bronze is simultaneously wicking into the part. This is called a one step process because sintering AND infiltration happens in a single firing cycle. It should be understood that overall design and geometry is a prime factor in how well any part will print and hold tolerances. In a few rare cases I have seen parts that required a temporary support cage to be printed in order for successful results. But generally for most small objects (Shapeways parts) if attention is given to the use of fillets and clean transitions good parts can be consistently produced. I have seen a bunch of propellor impeller designs that have come out beautifully. Even some working prototype turbo impellers. So when you draw up your prop just make sure the transition from the hub to the blade is filleted or blended smoothly. Avoid any hard right angles. Even castings and forgings require this detail to create strong functional parts.
Sorry for the long winded explanation but hopefully, you now understand a bit more about 3dmp.
-G

Thanks!
One more question- Is it possible for me to define where the stilt(s) are attached? Clearly, some areas are better for filling... and some areas are best left without cut-off gates...

PM

GlenG -- Thank you so much for that lucid and extremely informative description. My whole mental model of the process just got a major update. Cheers to you.

PM,
Shop techs usually make the decision about stilt placement. Generally there is no trace left of where the stilt had been. If you have a critical concern about this issue you will need to contact a SW rep.

-G

ah well, it's moot.

I dreamed of sourcing model boat props from this process... but the raw blank is 3x the cost of props hand cast, polished, and finish machined in the UK and retailed in the US!

Back to the drawing board...

And thank you, specifically, Glen... for mentioning that the sintering is done in a vacuum oven.

I've considered trying to use Shapeways SS parts in an HV-lowUHV vacuum system for research work... but had been worried about air bubbles and so forth, expecting porosity. Presumably I'm still not completely safe in that regard, but with (say) a decent bakeout of the Shapeways part, and a nice long pump-out of any virtual leaks, it sounds like it might well be workable.